The present invention relates to a flotation machine for deinking in which ink and oil pitch contained in waste papers are attached to bubbles, said flotation machine being used in a waste-paper recovery installation for defiberizing and cleaning waste papers into papermaking materials.
Deinking methods for waste papers are in general divided into flotation method, washing method and combination thereof.
In the flotation method, air is admixed to waste-paper stock liquid, which has been obtained by defibering waste papers and adding chemical agents to them, and free ink particles are adsorbed by air bubbles generated. After the air bubbles having the ink particles entrained therein float up to the surface of the liquid, the ink particles are separated and removed. On the other hand, in the washing method, a large quantity of water is forced to flow thereby to remove free ink particles.
From the viewpoint of obtaining satisfactory results in the washing method and suppressing lowering of the yield and increase of load in a effluent water system, combination of flotation method with washing method has been mostly used. If the flotation method is strengthened, the washing method may be eliminated or minimized.
In the flotation method, free ink particles in the waste-paper stock liquid are adsorbed to the bubbles and float, whereby the ink particles are separated and removed from the liquid. The smaller the diameters of the bubbles are, the more readily the fine ink particles are adsorbed to them. It is therefore important for efficiently carrying out the flotation method that the total surface of the bubbles is large; the bubbles are uniformly distributed in the waste-paper stock liquid; and residence time during which the bubbles are remaining in the stock liquid is longer. When the same quantity of air is admixed to the stock liquid, the finer the bubbles are, the larger the total surface of the bubbles becomes and the slower the flotation speed of the bubbles becomes. The processing by the flotation method is therefore evaluated by how to uniformly admix a larger quantity of fine air bubbles to the stock liquid and how to effectively remove them as froth from the liquid.
Formerly, printing ink was weak in adhesion to paper fibers and was relatively readily separatable. Demand in quality for deinked pulp was not so high. As a result, former flotation machines could attain satisfactory results by a relatively small quantity of air and the shorter residence time of bubbles.
Recently, there arise many problems which can be hardly be solved by the former deinking technique using the older flotation methods. Firstly, adhesion strength of printing ink has been increased in accordance with developments of printing technique such as use of offset process in printing newspapers so that the mechanical force is required for separating the ink, resulting in separated ink particles being of finer diameters. Secondary, degree of utilization of waste papers has been increased since natural resources have been decreasing globally. Thirdly, demands in quality for paper used in printing have been enhanced because of enhanced demands for visual gorgeousness and coloration of printed papers. Fourthly, strict regulations have been applied to effluent water discharged from papermaking processes.
When the operation of the washing method itself is intensified, effluent water cannot satisfy strict regulations. It is therefore a general acceptance among experts in the art that selection of effective flotation method is a best approach for overcoming the above described problems. Thus, various powerful flotation machines have been devised and demonstrated.
A recent trend in flotation method is such that cell volume is increased and residence time is prolonged to thereby increase opportunities of contact of bubbles with ink particles. Air bubbles are made finer and are admixed in a large quantity to waste-paper stock liquid to produce and remove a large quantity of froth. Such trend is influenced not only by mechanical improvements but also by improvements of deinking agents (surfactants).
Table 1 shown below illustrates comparison in mechanical performances between a former flotation machine and a recent flotation machine.
TABLE 1 ______________________________________ Former Recent machine machine ______________________________________ Air volume (G/L)* 1-3 4-8 cell number per 6-20 3-6 100 pulp tons per day Cell capacity (m.sup.3) per 70-120 150-200 100 pulp tons per day Cell capacity (m.sup.3) 5-20 20-50 Primary residence time** (min) 10-15 20-30 Power source unit 20-40 40-60 (kWH per pulp ton) Increment in brightness (Hunter) 3-4 6-10 ______________________________________ Remarks: *G/L: total air quantity (Gm.sup.3 /min) per total processing liquid (Lm.sup.3 /min) **residence time in primary flotation machine. Primary reject is secondarily processes and secondary accept is returned to primary inlet.
FIG. 1 illustrates an example of former type vertical cylindrical cell flotation machines in which reference character a denotes an air inlet; b, a stock inlet; c, a stock outlet; d, a cell; e, an air port for pushing out froth; f, a froth fan; and g, a froth outlet. As shown, two cells substantially identical in construction are stacked. Air is admixed to the stock liquid by means of an ejector effect at the stock liquid inlet. The liquid is made to tangentially flow into the cell and swirls around the axis of the cell with the air admixed to the liquid. In the cell, the air floats up as bubbles to form froth. The stock liquid is discharged from the vicinity of the axis of the cell via the outlets c. Froth is forced to flow into an opening (not shown) at the froth outlet g by the air blown from the port e on the side wall of the cell near the liquid level and then is discharged through the outlet g to the exterior. The froth pushing air is forced to cycle by the froth fan f.
In the flotation machine of this type, the quantity of air sucked cannot be increased since the air is sucked by means of the ejector effect. Even if the air is compressedly admixed, the result is merely that larger-diameter ineffective bubbles are increased in quantity. In addition, it is difficult to optimumly control the velocity of the swirling liquid; if the velocity of the liquid is too low, stock rejects may be increased; if the swirling velocity is excessive, the fine air bubbles, which has less floating force, may fail to cross the swirling flow to float up, disadvantageously resulting in unsatisfactory separation of the froth from the stock flow.
FIG. 2 illustrates, as another example of the former type machines, a box cell flotation machine in which reference numeral h denotes an air inlet; i, a stock inlet; j, a box type cell; k, a discshaped impeller having a plurality of blades; m, a stock outlet; and n, a froth outlet. The flotation machine of this type is generally called "Denver" type. The stock liquid is introduced to the center of the impeller k while the air is sucked by itself. The liquid admixed with the air is diffused along the bottom surface of the cell due to the centrifugal force of the impeller k and rises along the side surfaces of the cell. At the liquid level, the air bubbles are separated as froth from the stock liquid. The liquid separated flows down along the center portion and circulates. Part of the liquid is discharged through the outlet m while the froth is discharged through the outlet n.
In such box cell flotation machine, the impeller k must be rotated fast so as to produce fine air bubbles; but if its speed is too fast, the circulation of the liquid in the cell may become too violent, disadvantageously resulting in the floating froth swirled back into the stock liquid.
The flotation machines of the types described above are defective in that the air for producing fine air bubbles cannot be increased in quantity because of the machines being of self air suction type. Even if the air were forcibly introduced into the machine, only the quantity of ineffective air would be increased and satisfactory result could not be attained. Furthermore, because of insufficient mixture and separation of the air/liquid, cells must be stacked in stages in series, resulting in complicated installation.
By contrast, in recently developed flotation machines, high rotational speed of the rotor causes air bubbles to receive strong shearing forces to be converted in a larger quantity into fine air bubbles, which are admixed to the stock liquid. Agitating action is intensified to satisfactorily diffuse air bubbles in the liquid and to increase the residence time of air bubbles in the liquid to thereby increase opportunities of the ink particles being made contact with the liquid. The cell is of large capacity to afford a sufficient period of time permitting floating and gathering of air bubbles. These are the features of the recently developed flotation machines.
FIGS. 3 and 4 illustrate a rotary diffusion type flotation machine as an example of the most recently developed machines (Japanese Patent 1st Publication No. 245390/1986) in which reference numeral 51 denotes a vertical cylinder type cell; 52, 53 and 54, weirs; 56, rotary diffusion pipes; 57, a rotating shaft; 58, an air supply inlet; 59, a liquid level; 60, a stock supply inlet; 61, a stock outlet; 62, a rotary type froth collection blade; and 63, a froth trough.
The stock liquid flows through the inlet 60 into the cell 51 and repeatedly flows up and down in a zigzag manner between the weirs 52, 53 and 54 and then is discharged through the outlet 61. The air flows through the inlet 58 into the diffusion pipe 56 rotating at a high rotational speed and flows into the stock liquid through vent holes 20-40 mm in diameter opened through small projections 64 on the pipe 56. Because of difference in velocity between the liquid and the surface of the diffusion pipe 56, the air receives strong shearing forces to become fine air bubbles, which are diffused into the liquid. The stock liquid is violently agitated by the rotating force. The air bubbles in the liquid float up to the liquid level 56 to form froth, which is collected by the scraping blade 62 into the trough 63 and discharged to the exterior. The discharged froth may be secondarily processed as needs demand.
In the flotation machine of this rotary type, in order to produce fine air bubbles and agitate the liquid in the vessel having a large capacity, the large-diameter diffusion pipe 56 must be rotated at a high rotational speed so that powerful driving force is required. The stock liquid repeatedly flows up and down between the weirs 52, 53 and 54 and the bubbles in the downward flow are difficult to rise up, adversely affecting the the separation of the bubble from the liquid; as a result, production of froth over the liquid level varies between the liquid surface portions where the upward flowing liquid appears and those where the liquid flows downwardly. Since the air is injected into the liquid from interiors of the diffusion pipes 56, the liquid-tight sealing 65 between the pipe and the wall of the cell is complicated and its maintenance is difficult. When the floation machine is started or stopped or the balance between the air pressure and the liquid pressure is lost, the stock liquid may flow into the diffusion pipe 56 and adhere to the interior wall thereof due to the centrifugal force, thereby clogging the air vents on the wall of the pipe 56. Agitated flows produced by the diffusion pipe 56 are unstable so that when variations in concentration or the like of the liquid occur, the flow rates of the stock liquid may vary and the air bubbles may suddenly burst.
The present invention was made in view of the above-described problems of the former or most recent machines and was based on the following concept and the results of the experiments conducted by the inventors.
When a certain volume of air is admixed to a liquid, the total surface area of bubbles is substantially in inverse proportion to mean diameters of the bubbles so that the smaller the mean diameter of the bubbles is, the larger the total surface area becomes. Velocity of air bubbles flowing upwardly is substantially in proportion to mean diameter of the bubbles so that the residence time of the bubbles in the liquid is substantially in inverse proportion to the mean diameter of the bubbles when the depth of the liquid remains constant. It follows therefore that the opportunities of the air bubbles being made into contact with the ink particles to adsorb and entrap them is substantially in inverse proportion to the square of the mean diameter of the bubbles so that the less the diameters of the bubbles are, the more remarkably the above-described opportunities increased.
According to experimental results, it is noted that the larger the volume of air is admixed and the larger the volume of the froth discharged is, the better the brightness is improved. FIG. 5 illustrates the relationship between the brightness (Hunter) of the accept stock and the flow-rate reject rate, obtained by processing the waste-paper stock consisting of 100% offset-printed newspaper. It is noted that even when the flow-rate reject rate is increased in excess of 20%, the brightness is not substantially increased accordingly so that the flow-rate reject rate on the order of 15-20% is preferable (in this case, the fiber stock in the reject is lower in concentration than that at the stock inlet and stock reject rate is on the order of approx. 5%).
The present invention was made based on the above-mentioned problems encountered in the conventional flotation machines, conceptions of the inventors and results of the experiments conducted by the inventors and has the following aims:
(1) Fine air bubbles are uniformly admixed to the stock liquid, thereby eliminating the necessity of blowing an excessive volume of air into the liquid and thus reducing required power for blowing the air thereto.
(2) The air bubbles are uniformly admixed to the stock liquid by a less degree of agitating power.
(3) Turbulence and dead zones in the flow passages in the cell are eliminated to decrease cell capacity for a volume of stock liquid to be processed as well as variation in brightness in the accept.
(4) The opportunities of the air bubbles being mixed with and separated from the stock liquid are repeatedly given to thereby shorten the processing time and consequently decrease the cell capacity for a volume of stock liquid to be processed.
(5) Disturbance on the liquid level and variation in generated froth are eliminated to smoothly remove the froth without being swirled back into the liquid.
(6) Finer air bubbles are produced to remove finer ink particles and to increase the opportunities of trapping the ink particles. And,
(7) the air bubble generating means or device is made simple in construction and in maintenance and is reliable in operation.
To attain the above-described aims, the flotation machine for deinking in accordance with the present invention comprises a horizontally extending cylindrical cell having opposite end plates to define a reservoir for a stock liquid with a free surface at its top, a froth trough on a upper portion of said cell for receiving froth floating up to said free surface of the liquid and for discharging the froth to an exterior, a stock inlet at one end of said cell for supplying a stock liquid, a stock outlet at the other end of said cell for discharging the stock liquid, at least an air bubble generating means at a lower portion of said cell and horizontally extending between said end plates, whereby said stock liquid admixed with air bubbles from said air bubble generating means biasedly flowing in the whole interior of said cell from the stock inlet to the stock outlet in the form of a spiral with a horizontal axis, said air bubble generating means comprising a turbine rotor adapted to be rotated at a high rotational speed and having an air supply pipe disposed above and adjacent to said turbine rotor along a generating line of the rotor and having at least an air port opened toward said turbine rotor.
In the flotation machine for deinking in accordance with the present invention, the stock liquid flows into the cell at one end thereof and is sucked into the air bubble generating means or device where it is mixed with the air bubbles and is forced to flow not circumferentially uniformly but biasedly out of the air bubble generating means. The stock liquid admixed with the air bubbles rises within the cell along the spiral flow path.
The bubbles reach the free surface of the liquid together with the liquid flow and then separate from the liquid, remaining as froth over the free liquid surface. The froth overflows into the trough and flows out to the exterior.
The liquid now free from the bubbles flows down in the cell through the downward flow path oppositely of the rising liquid path and passes again through the air bubble generating means where the liquid again entrains air bubbles. Thus, the stock liquid repeatedly flowing up and down and finally flows through the stock outlet to the exterior of the cell.
The air bubble generating means or device will generate fine air bubbles according to the mechanism shown in FIG. 10. More specifically, the air from an air port of an air supply pipe above the rotor joins in the liquid circulating around each turbine blade moving at a high speed and then is entrapped in a negative zone on the rear side of the blade and flowing into the rotor. On the other hand, the stock liquid circulating in the vicinity of and in unison with the turbine rotor strikes on the air supply pipe and is suddenly decelerated to be increased in pressure so that part of the stock liquid flows into the turbine rotor, which also causes air to flow into the turbine rotor together with the part of the liquid. Air and stock liquid flowing into the turbine rotor receive the shearing forces by the inner edges of the blades and are mixed together. The liquid then flows out of the turbine rotor due to the centrifugal force so that the air bubbles entrained in the liquid are subjected to the shearing forces by the outer edges of the blades to become finer and be uniformly distributed in the stock liquid.
Most of the stock liquid circulating in the vicinity of and in unison with the turbine rotor moves away from the rotor into the rising flow, which facilitates formation of the spiral flow path in the cell.
The present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.